Safety control arrangement

ABSTRACT

A safety control arrangement which is adapted for use with fuel burning appliances such as natural gas fueled fire place logs or the like, and which is responsive to draft conditions in the venting system, so that fuel flow to the burner will be cut off in the absence of predetermined air flow characteristics. The unit preferably includes a probe or detector placed in the air stream and operatively associated with a pair of switches so as to detect both lack of vacuum induced by normal draft and excessively high vacuum, indicating vent blockage upstream of the vent fan. The switches in the electrical control circuit are arranged so as to keep the fuel valve open only when a predetermined minimum draft is present, and to open the circuit when insufficient or excessive vacuum is present, regardless of the cause or location of vent blockage or restriction. In the preferred form, a parallel vacuum connection is made between the detector and the low threshold and high threshold switches, and a series electrical connection is made between the fuel valve and the normally opened and normally closed electrical switches actuated by vacuum in the vent passage.

United States Patent [1 1 Mueller et al.

[ Oct. 30, 1973 SAFETY CONTROL ARRANGEMENT [76] Inventors: Ronald J. Mueller, 14856 S.

Y Trumbull, Midlothian, 111. 60445; Verl Sinko, 3214 l-lalsted, Steger, 111. 60475 [22] Filed: Mar. 31, 1972 [21] Appl. No.: 240,194

Primary ExaminerCarroll B. Dority, Jr. Attorney-James T. Fitzgibbon and Wetzel, Gre'enawalt & Fitzgibbon [57] ABSTRACT A safety control arrangement which is adapted for use with fuel burning appliances such as natural gas fueled fire place logs or the like, and which is responsive to draft conditions in the venting system, so that fuel flow to the burner will be cut off' in the absence of predetermined air flow characteristics. The unit preferably includes a probe or detector placed in the air stream and operatively associated with a pair of switches so as to detect both lack of vacuum induced by normal draft and excessively high vacuum, indicating vent blockage upstream of the vent fan. The switches in the electrical control circuit are arranged so as to keep the fuel valve open only when a predetermined minimum draft is present, and to open the circuit when insufficient or excessive vacuum is present, regardless of the cause or location of vent blockage or restriction. In the preferred form, a parallel vacuum connection is made between the detector and the lowthreshold and high threshold switches, and a series electrical connection is made between the fuel valve and the normally opened and normally closed electrical switches actuated by vacuum in the vent passage.

8 Claims, 5 Drawing Figures SAFETY CONTROL ARRANGEMENT BACKGROUND OF THE INVENTION ture. Commonly, units of this type are installed within' fireplaces in homes and apartments, with the fireplaces being constructed either during or after the house, apartment or other structure is built. The fireplaces or appliances contain one form or another of passage or vent to the outside, such vent of flue ordinarily being a duct or pipe extending through the side wall of the building structure. Draft for removal of vapors is accomplished by providing an exhaust fan in the vent, commonly adjacent the outlet thereof. At the end of the vent pipe, doors are usually provided which remain normally closed in the absence of a draft but which can be opened by the force of the air flowing through the vent under the influence of the fan.

ln constructions of this type, it is necessary that the structure be vented in a substantially foolproof manner, and that the draft or ventilation system be characterized by simplicity, and, most importantly, reliability.

in the past, this has generally been accomplished by three different methods, as will now be set forth.

The first prior art method which is commonly used employs a thermostatic switch which is responsive to unduly elevated temperatures which might be encountered if the burner is vented improperly. This system is sensitive to unduly high temperatures in the combustion area. However, these units have not proved reliable, because, among other things, they lack sensitivity, and because, especially in the case of smaller burners, there can be a substantial temperature variation in' areas closely adjacent the burner whether or not the vent is functioning properly. Accordingly, units of this type are not capable of the extremely high degree of reliability necessary for completely safe operation under all reasonably foreseeable conditions of use.

The next common prior art method has included the use of centrifugally responsive switch elements associated with vent fans or fan motors. in constructions of this type, the main gas valve to the burner is kept open as long as the fan continues to spin. Although units of this type desirably shut off gas flow if the fan stops rotating for any reasons, they are not effective where the fan continues to spin, but wherein the flue or vent is nevertheless otherwise blocked off. Thus, if the vent doors downstream of the fan became locked shut for any reason, of if the flue became partially or completely blocked either upstream or downstream of the fan, the fan would continue to rotate and the rotationsensitive switch would keep thegas valve open, thus permitting a dangerous conditionto continue.

The third common safety device which has been used is that of the so-calied sail switch which is disposed within the vent and in which a light weight element sails" upwardly in response to a draft in the flue, thereby indicating the presence of a draft therein. Al-

though theoretically satisfactory in some respects, switches of this type utilize extremely thin membranes or filaments which, under conditions of use in a flue, often become covered with carbon and are rendered inoperative. Thus, this type of switch is too delicate for reliable use over a prolonged time period. Moreover, such switch might also be kept open and permit gas to flow in the burner if there is onlypartial flue blockage downstream of the flue and the switch. In this case, although definite air flow would be present, an undesirably restricted volume of venting air would be available, and an actually or potentially dangerous condition would result.

As a result of the situation just described, there are no presently known safety switch devices or arrangements which 'are considered safe enough 'to be approved for the American Gas Association for use with burners of the type in question.

The present invention, in contrast to prior art developments, provides a system which, in order'to keep the burner in operation, requires the existence in the flue itself at all times of a moderate vacuum, suction, or draft level which is characterized by air flow of a predetermined velocity, and which is arranged to interrupt gas flow in the event blockage occurs either upstream or downstream of the fan, regardless of whether or not the fan is turning, and which will detect partial as well as complete flue blockage, all of the above being independent of temperature conditions surrounding the burner.

In view of the foregoing shortcomings of the prior art, and the need for an improved safety control arrangement for gas-burning, vented appliances, it is an object of the present invention ro provide an improved burner safety switch.

Another object is to provide a switch assembly for use with an appliance to which fuel flow is controlled, with the switch being responsive to normal draft vac,- uum, as well as to uncommonly high or low draft vacuum levels, and which will shut off the fuel supply to the controlled appliance in response to excessively'high or low vacuum in the flue or vent used with the appliance.

Another object is to provide a control and switching arrangement for fueled appliances wherein the switch has, in effect, a responsive band width which may be varied over a considerable range and which will provide positive switching action on either side of a neutral band.

Another object is to provide a safety switch assembly which utilizes a parallel condition-detecting arrangement and a series switching control arrangement, whereby a pair of responsive elements alternatively detect two different conditions, and whereby both undersirable conditions must be positively eliminated in order for the control to permit continued combustion at the burner.

Another object is to provide a safety device having means for creating a vacuum in the presence of a predetermined air flow, means for detecting a vacuum at a predetermined first level,'means for detecting a vacuum of a second, predetermined higher level, and a series switching arrangement between the detectors, whereby the presence of at least the first vacuum level and the absence of the second vacuum level must be concurrently present to permit continuedfuel flow to the combustion appliance.

A still further object is to provide a detector and switch arrangement for use in appliances and .which may be conveniently located in a position remote from the burner but easily accessible for service.

A further object is to provide a detection and switch assembly which is economical to manufacture and reliable in use.

Still another object is to provide a safety switch unit which, to a large extent, uses readily available elements of established dependability and reliability.

Another object is to provide a switch assembly which includes a detector in the form of a probe having means for creating a sensible vacuum in the presence of a predetermined level, of air flow, and means for connection between the probe and switch units sensitive to predetermined vacuum or pressure levels.

A further object is to provide a safety switch unit I which is of equal sensitivity but of greatly increased reliability in relation to sail type switches used in the prior art for similar purposes.

A still further object is to provide a switch assembly which will detect reduced air flow of a predetermined extent independently of the operation of means intended to force the air through the vent.

Still another object is to provide a safety switch assembly having a vacuum detector, a first switch which is normally opened but movable to a closed position in response to a predetermined first minimum vacuum level, a second switch which is normally closed but movable to an opened position in response to a predetermined vacuum levelhigher than said first minimum vacuum level, and a control circuit including means for closing the fuel valve of an associated burner unit unless both first and second switches are closed.

Yet another object is to provide a control unit having a pressure detector, means for furnishing a signal in response to the detected pressure simultaneously to first and second switches, one of said switches causing a control circuit to close upon detection of a minimum effective flow rate in said vent, and the other of the switches to open in the presence of excess vacuum in said vent, a series control connection between the switches, means for biasing a fuel supply valve to a normally closed condition, and means responsive to a closed control circuit for opening said normally closed fuel supply valve.

The foregoing and other objects and advantages of the invention are achieved in practice by providing a detector unit having means associated therewith for creating a partial vacuum of a first level in response to air flow in associated vent, means for detecting a second, increased vacuum level in the vent in case of obstruction thereof, means for completing a control circuit only in the presence of a vacuum level between such first and second levels with the control circuit being adapted to permit fuel flow in an associated appliance only in the closed or completed state thereof.

The manner in which these objects and advantages and other inherent objects and advantages are achieved in practice will be better understood when reference is made to the following detailed description of the preferred embodiments of the invention described in detail and shown in the accompanying drawings, wherein like reference numbers indicate 7 corresponding parts throughout.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view, with portions in elevation, showing a burner unit disposed within a fireplace and having a gas flow control switch operatively associated with the novel safety control of the present invention;

FIG. 2 is an enlarged perspective view of the vacuum probe or pressure level detector of the invention;

FIG. 3 is an alternate form of detector;

FIG. 4 is a schematic view of a preferred form of electrical circuit useful with the invention; and

' FIG. 5 is an enlarged vertical sectional view of an alternate form of the safety control apparatus of the invention, showing portions of the unit to be mounted exteriorly of the wall through which the unit-is vented for easy access to the components thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION While the present invention is capable of use in different environments and applications, certain preferred embodiments thereof will be described wherein the unit is used in the vent or flue of gas-fired fireplaces or burners directly exposed to the interior of a room, wherein positive air venting is achieved by the provision of an electrical fan, wherein the remote switches are diaphragm type switches directly operated by air pressure, wherein the fuel is natural gas, and wherein the control circuit includes electrical switch having normally opened and normally closed contacts and wired in series with the fuel control valve. 7

Referring now to the drawings in greater detail, FIG. I shows a fireplace assembly generally designated 10 to be disposed within a room area 12 which is partially defined by an exterior wall 14, and shows that the fireplace includes one or more gas burner units 16 which may be mounted as shown at 18 on the floor or hearth 20 of the fireplace. A gas line 22 extends between a gas control valve 24 and a service pipe 26 which is adapted to receive natural gas or other suitable fuel from a supply thereof in a known manner. A hood unit generally designated 28 is disposed above the open portion 20 of the fireplace 10 in which the burner 16 is received, and the interior of the hood 28 includes a vent passage 30 having an exterior portion 32 thereof extending through the valve 14, as well as an intermediate portion 34, an elbow 36 and a horizontal section 38.

In accordance with one form of the invention, means in the form of a fan 40 driven by a motor 42 are provided for creating forced air circulation, and accordingly, a draft within the vent passage 30. A plurality of lightweight, swinging doors 44 are disposed at the outlet end of the exterior portion 32 of the vent, and in a known manner, these doors 44 are opened by the force of the draft created at least in part by the fan 40 when the fireplace 10 is in use. The hinged doors 44 are arranged so as to fall by the force of gravity into a closed position when, for one reason or another, there is no substantial air flow in the passage 30. Ordinarily, this serves to prevent back flow or outside air into the interior of the room 12.

In keeping with the invention, a novel safety switch assembly generally designated 46 is shown for purposes ofillustration to be mounted as by a flange 48 just out- 7 side the central portion 34 of the passage 30. The assembly 46 includes a pressure or vacuum level sensor or detector 50 disposed within the vent passage 30, and includes tubing 52 branched at a tee 54 into a pair of oppositely directed lines 56, 58 which terminate respectively in an operative connection to pressure or vacuum responsive means in the form of air operated diaphragm switches 60, 62.

In the form shown, the switch 60 is a normally open switch which closes at a first, relatively low pressure or vacuum level, preferably having a threshold of about 0.15 inches of water. The second switch 62 is a normally closed switch which is biased somewhat more strongly to a closed position but opens at a preset value, typically 0.3 to 0.4 inches of water. Disposed within each of the switches 60, 62 is a pair of electrical contacts of conventional type.

Wiring to the switch is arranged so that a current from a source (not shown is fed through line 64 to one terminal of the switch 60, then through another line 66 to one terminal of the second switch 62, and finally, through wire 68 from the other terminal of the switch 62 to means in the form of a solenoid operated switch 70 associated with the fuel control valve 24. The function of these electrical conductors and switches will be discussed in detail elsewhere herein.

Referring now to FIG. 2, details of the probe 50 are shown and it may be seen that the probe includes a pair of flanges 72, 74 meeting along a line 76, and defining therebetween a groove 78 in which is disposed a tube 80 having a closed end portion 82 and a plurality of spaced apart openings directed downstream of the air flow direction, that is, upwardly as shown'in FIG. 2. The flanges 72, 74extend outwardly from a vertically extending mounting element 86, which may be a separate structure or which may be integrally formed from an interior surface of the vent passage 30.

The interior of the tube 80 forms a lowpressure of vacuum line or chamber which communicates with the interior of the line 52, which in turn communicates, as is best shown in FIG. 1, with each of the switches 60,

Referring now to FIG. 3, an alternate form of detector 50 is shown to include the flanges 72a, 74a meeting along line 76a to define the groove 78a; however, in this embodiment, the tube 80a is in the form of a short stub having an open end 82a rather than having the closed end 82 shown in FIG. 2.

Referring now to FIG. 4, which is a schematic illustration of an electrical circuit, it will be noted that the electrical line 64 preferably extends from a schematically illustrated current source 88 to the normally open and normally closed pairs of contacts 90, 92, respectively, and that a line 66 extends between the pairs of contacts 90, 92 while line 68 extends to the solenoid 70 associated with the gas valve 24. All of the foregoing connections are in series and are shown, merely for purposes of illustration, to include a line 94 which may be grounded, or which may be a return line, as shown in phantom lines, connected with the current source Referring'now to the operation of the unit, it will be assumed that the user is prepared to light the burner 16 which may be considered for purposes of illustration to be lighted with a match or taper. When a manually operable starter valve (not shown) is pressed, the circuit is actuated and while gas flows into the burner 16 for ignition, the motor 38 is energized and the fan blades revolve, creating a perceptible or sensible air flow 6 within the passage 30, such air opening the doors 44 and passing into the atmosphere exterior to the wall 14.

' This air draft over the flanges 72,,74 forming the V- shaped trough 78 creates a low pressure area upstream of the V defined between the flanges 72, 74 and causes a partial vacuum or reduced pressure level within the trough 78 and accordingly, within the tube 80. According to the invention, this vacuum is of a level which is sufficient to cause. diaphragm actuation and to close the contacts 90 of the normally open switch 60. This permits current to flow in the lines 64, 66, 68 of the control circuit and actuates the solenoid 70 which keeps the gas valve 24 open. This is the normal condition of affairs during use of the fireplace 10.

However, assuming that, for one reason or another, such as because of freezing, lack of lubrication, or 0th erwise, the doors 44 fail to open. In such a case, since no substantial net air flow exists in the passage 30, the pressure adjacent the openings 84 in the tube 80 will rise (vacuum will be .lost) and the normally opened contacts 90 will be opened,' de-energizing the control circuit and shutting off fuel flow through'the valve 24. Assuming that the doors 44 are operable, but that the fan 40 fails to operate, regardless of the reason therefor, again'no net air flow will occur in the passage 30 and switch contacts 90 will again open, cutting off air flow.

Assuming now that the fan 40 and doors 44 are operating, and that there is no obstruction downstream of the probe 50, but assuming that there is an at least partial obstruction of the passage 30 upstream of the probe 50, the fan 40 will continue to operate, but because of reduced air accessibility to the passage 30, a relatively high vacuum or relatively very low pressure area will exist in the region of the probe 50. In this connection, it will be understood that this increased partial vacuum may exist when there is no net air flow in the passage 30, by reason of complete blockage thereof, but may also occur when there is air flow of a reduced volume. In either case, assuming the level to exceed the threshold level of the switch 62, the normally closed contacts 92 thereof will be opened, thereby de-energizing the control circuit and the solenoid 70 and permitting the valve 24 to close, shutting off gas flow to the burner 16. In the preferred form of unit, the switches are set so that a certain minimum air flow is required to open the contacts while a predetermined but less than complete degree of blockage of the passage 30 will cause sufficient vacuum in the switch 62 to open the normally closed contacts 92. The 0.15 and 0.3 to 0.4 inches of water level have been found satisfactory to permit detect'ion of acceptable minimum air flow and maximum suction; however, these values are arbitrary, and any operable limits may be used.

Referring now to the results achieved by the present invention which were not achieved in practice by prior art constructions, it will be noted that any switch, such as a centrifugally operated mercury switch which is responsive only to fan movement, could not be completely effective, since such an arrangement is inherently incapable of detecting certain conditions of blocked air flow, whether such blockage occurs upstream or downstream of the fan. That is, the fan blade may be rotated without producing a venting draft.

Likewise, it will be appreciated that a sail type switch, although commonly used even without a safety approval, is not fully satisfactory. This is because, aside from the delicacy and inherent unreliability of said switches, which is brought about by the present disposition or susceptibility thereof to becoming choked or clogged with carbon particles, such switches cannot detect a condition such as would occur upon partial blockage of the air flow upstream of the detector. Thus, a partially blocked flue or vent would result in a higher flow rate of a reduced volume of air, thereby keeping the switch open and failing to recognize that an insufficient volume of air is being provided to vent the unit. Since a number of the gas burners with which the invention is intended for use are those which burn pure gas rather than an air-gas mixture, reduced venting air flow in the flue is particularly hazardous.

From the foregoing description, it can also be appreciated that thermocouple detectors are likewise not satisfactory for discovering the presence of all hazardous operating conditions. Thus, while a thermocouple is satisfactory to shut off the gas flow if the fire is extinguished, and whereas a more sensitive thermocouple or switch might be satisfactory for detecting unduly raised temperatures in the vicinity of the fire, or in the flue or vent passage, such units likewise do not detect conditions of oxygen starvation caused by downstream vent blockage. As a result of the foregoing, no known prior art system, particularly those available at reasonable cost, has been developed for reliable detection of all potentially hazardous venting conditions which may occur in the use of this type of applicance.

Referring now to FIG. 5, a form of the invention is shown in a fragmentary view wherein the detector unit 46b is placed exteriorly of the wall 14b through which the passage b extends. In the form shown, the detector or probe 50b includes the V-shaped flanges with the edge portion 76b thereof facing opposite the direction of air flow, that is, with the apex upstream of the remaining elements of the probe. In the embodiment of FIG. 5, the tube 80b is continuous and extends between switches 60b, 62b which are shown to be disposed on opposite sides of the passage 30b and exteriorly thereof. The fan and its associated motor 42 are disposed within a housing 100 which includes the doors or shutters 44b of the type shown in FIG. 1. Although not shown in detail, it will be appreciated that the wiring of the unit is the same, that is, leads 64b, 66b, and 68b respectively, extend to one terminal of the switch 60b between neutral terminals of the switches 60b, 62b and to the control for the fuel value, respectively. In use, the embodiment of FIG. 5 operates in the same way as the embodiment of FIG. 1, since the diaphragms of the switches are operated by a parallel connection while the electrical circuit comprises a normally on and a normally off switch in series relation.

Although FIG. 5 shows the switches 60b, 62b to be vertically spaced apart, it will be understood that it is sometimes preferred to dispose the switches 60, 62 horizontally with respect to each other and place the detector b in the bight portion of a U tube which is also formed by two vertically extending legs communicating at the bottom ends thereof with the perforated detector tube 80 and communicating at their upper ends with the switches 60, 62.

Likewise, if a stub form of detector such as that shown in FIG. 3 is used, the flanges may extend vertically into the vent 30, and the tube 80 may, be connected to a tee joint serving the diaphragms of both switches.

Referring now to other features of the invention, and

in particular to the components and materials which are preferred for use with the invention, the diaphragm operated electrical switches of the type shown at 60 and 62 are readily available commercially. Likewise, no special materials are required to form the probe or detector, although copper or steel tubing and steel flanges are preferred for use in construction of the probe. Plastic or metal tubing is suitable to form the vacuum or pressure connections between switches, and the wiring thereof is otherwise conventional. The safety control switches and the like are preferably held within a metal or other housing which is mounted to the vent arrangement by metal screws or otherwise, with an opening being provided in the vent and with the probe having a gasket (not shown) to facilitate relatively airtight insertion of the probe into the air stream.

By upstream and downstream as used herein is meant the direction of air flow under the force of blower or fan, and reference is not intended to be made to the physical direction or orientation of air flow. The words vacuum and pressure are used herein in their normal sense, that is, a higher vacuum level refers to a lower pressure, it being understood that, under normal draft conditions, there will be reduced air pressure'or increased vacuum within the vent under normal conditions, and further reduced pressure or higher vacuum level where there is a blockage in the vent upstream of the air propulsion unit.

Although the described arrangement is advantageous because of the highly reliable nature of the components used therewith, even further reliability may be obtained without prohibitive cost by providing one or more additional, identical control arrangements, with all the electrical switches in series with one other, whereby failure of one or more switches in any one arrangement would not result in holding the fuel valve open, where the switches in any other arrangement remained operative.

The invention has been described in detail with reference to a vacuum actuated electrical control circuit associated with a gas valve, however, it is anticipated that mechanical or fluidic controls might be used as well as electrical controls, the exact fonn of control used not being essential to the invention. Moreover, the invention is susceptible of use with other safety devices intended for other purposes, such as thermocouple detectors or the like adapted to shut off gas flow when the flame is extinguished.

From the foregoing, it will be seen that the present invention provides a new and improved safety control unit especially adapted for use with fuel burning applicances, such unit having a number of advantages and characteristics, including those pointed out herein and others which are inherent in the invention. Various embodiments of the invention having been set forth by way of example, it is apparent to those skilled in the art that modifications and changes may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

We claim:

1. A safety control unit for use with a fueled combustion appliance, said unit comprising, in combination,

means for detecting the air pressure level in a combustion vent, first means operatively associated with said detector and responsive to a first, reduced pressure level characteristic of normal air flow in said vent, second means-operatively associated with said detector and responsive to a substantially further reduced, second pressure level in said vent, and a control circuit having a normally open portion thereof, operatively associated with said first means and adapted to be closed when the pressure in said vent is reduced to said first level, said control circuit also having a normally closed portion, said normally closed portion being operatively associated with said second means and being adapted to be opened upon detection of said second pressure level in said vent, said control circuit including a portion adapted for connection to a fuel flow-control valve associated with said combustion appliance, whereby flow of fuel to said appliance may be controlled by the pressure level in said combustion vent.

2. A control unit as defined in claim 1 wherein said means for detecting said air pressure level comprises a probe adapted for insertion into said combustion vent, said probe including means for diverting air flow therearound to produce a reduced pressure level adjacent one portion thereof, said operative connection between said detector and said pressure responsive means being in the form of a tube extending between both of said pressure responsive means and said probe.

3. A control unit as defined in claim 1 wherein said detecting means is in the form of a probe having a pair of flanges inclined with respect to each other and joined along one common edge to define a generally V-shaped trough, and means disposed at least partially within said trough and defining a portion of a pressure line adapted for communication with said first and second pressure responsive means.

4. A control unit as defined in claim 1 wherein said control circuit is an electrical control circuit.

5. A control unit as defined in claim 1 wherein said first and second pressure responsive means include vacuum-sensitive diaphragms forming a part thereof.

6. A control unit as defined in claim 1 wherein said means responsive to said reduced pressure levels and said normally open and normally closed portions of saidcontrol circuit comprise a pair of diaphragm operated, pressure sensitive electrical switches forming a part of said control circuit.

7.'A control unit as defined in claim 1 wherein said operative association between said detector and said first and second means comprises a single pressure line extending from said detector to a line communicating with both said first and said second pressure-responsive means.

8. In combination, a gas combustion burner unit, a combustion vent for said burner, a gas valve for controlling fiow of gas to said combustion unit, an electrical control unit forming a part of said gas valve and being adapted to open and close said valve in response to conditions in an associated electrical control circuit, a normally open and a normally closed electrical switch forming a part of said electrical control circuit and arranged in series circuit relation, means disposed within said combustion vent for detecting the pressure level therein, means adapted to close said normally open switch upon detection of a first, predetermined vacuum level in said vent, and means associated with said second switch for opening said switch in the presence of a second, higher vacuum level in said associated vent, whereby said series circuit is completed only when the pressure level in said vent is between said first and second pressure levels. 

1. A safety control unit for use with a fueled combustion appliance, said unit comprising, in combination, means for detecting the air pressure level in a combustion vent, first means operatively associated with said detector and responsive to a first, reduced pressure level characteristic of normal air flow in said vent, second means operatively associated with said detector and responsive to a substantially further reduced, second pressure level in said vent, and a control circuit having a normally open portion thereof, operatively associated with said first means and adapted to be closed when the pressure in said vent is reduced to said first level, said control circuit also having a normally closed portion, said normally closed portion being operatively associated with said second means and being adapted to be opened upon detection of said second pressure level in said vent, said control circuit including a portion adapted for connection to a fuel flow control valve associated with said combustion appliance, whereby flow of fuel to said appliance may be controlled by the pressure level in said combustion vent.
 2. A control unit as defined in claim 1 wherein said means for detecting said air pressure level comprises a probe adapted for insertion into said combustion vent, said probe including means for diverting air flow therearound to produce a reduced pressure level adjacent one portion thereof, said operative connection between said detector and said pressure responsive means being in the form of a tube extending between both of said pressure responsive means and said probe.
 3. A control unit as defined in claim 1 wherein said detecting means is in the form of a probe having a pair of flanges inclined with respect to each other and joined along one common edge to define a generally V-shaped trough, and means disposed at least partially within said trough and defining a portion of a pressure line adapted for communication with said first and second pressure responsive means.
 4. A control unit as defined in claim 1 wherein said control circuit is an electrical control circuit.
 5. A control unit as defined in claim 1 wherein said first and second pressure responsive means include vacuum-sensitive diaphragms forming a part thereof.
 6. A control unit as defined in claim 1 wherein said means responsive to said reduced pressure levels and said normally open and normally closed portions of said control circuit comprise a pair of diaphragm operated, pressure sensitive electrical switches forming a part of said control circuit.
 7. A control unit as defined in claim 1 wherein said operative association between said detector and said first and second means comprises a single pressure line extending from said detector to a line communicating with both said first and said second pressure-responsive means.
 8. In combination, a gas combustion burner unit, a combustion vent for said burner, a gas valve for controlling flow Of gas to said combustion unit, an electrical control unit forming a part of said gas valve and being adapted to open and close said valve in response to conditions in an associated electrical control circuit, a normally open and a normally closed electrical switch forming a part of said electrical control circuit and arranged in series circuit relation, means disposed within said combustion vent for detecting the pressure level therein, means adapted to close said normally open switch upon detection of a first, predetermined vacuum level in said vent, and means associated with said second switch for opening said switch in the presence of a second, higher vacuum level in said associated vent, whereby said series circuit is completed only when the pressure level in said vent is between said first and second pressure levels. 